Method of coating an electrode

ABSTRACT

The novel anode may be used in the electrolysis of an aqueous solution such as of alkali metal chloride. The anode includes a base member having a conductive coating or surface comprising an oxy-compound of a platinum group metal such as ruthenium and an alkaline earth metal, typically calcium, or a rare earth metal such as lanthanum.

United States Patent 3,234,110 2/1966 Beer Bernard J. Dewitt Akron, Ohio 811,615

Mar. 28, 1969 Oct. 26, 1971 PPG Industries, Inc. Pittsburgh, Pa.

Inventor Appl. No. Filed Patented Assignee METHOD OF COATING AN ELECTRODE 6 Claims, No Drawings US. Cl 204/290 F, 117/201, 117/222, 117/223, 252/521 Int. (1 B011: 3/06 Field of Search 204/290,

References Cited UNITED STATES PATENTS Primary Examiner-F. C. Edmundson Attorney-Chisholm and Spencer ABSTRACT: The novel anode may be used in the electrolysis of an aqueous solution such as of alkali metal chloride. The anode includes a base member having a conductive coating or surface comprising an oxy-c'ompound of a platinum group metal such as ruthenium and an alkaline earth metal, typically calcium, or a rare earth metal such as lanthanum.

BACKGROUND OF THE INVENTION This invention relates to electrodes for electrolytic cells and, more particularly, to a corrosion-resistant, dimensionally stable anode such as for electrolysis of aqueous alkali metal chloride in the production of elemental chlorine or alkali metal chlorate.

The electrolysis of aqueous alkali metal chloride solutions such as solutions of sodium chloride or potassium chloride is conducted on a vast commercial scale. in the production of alkali metal chlorate, anodes and cathodes, or bipolar electrodes which when arranged in a spaced electrical series in an electrolytic cell may serve as both anode and cathode, are immersed in an aqueous solution of the sodium chloride or the like and an electrical potential is established between the electrodes. In the past, graphite or carbon electrodes have been used as anodes or as the bipolar electrodes in series. In consequence of the electrochemical reactions which occur, alkali metal chlorate is produced either directly in the cell or outside the cell after the solution is allowed to stand.

The electrolysis of alkali metal chloride to produce elemental chlorine and alkali metal hydroxide is conducted in two general types of cells-the diaphragm and the mercury cathode cell. In the diaphragm cell, the cell is divided into two compartments-which are separated by a porous diaphragm usually of asbestos. The cathode is of perforate metal and the asbestos diaphragm is in contact with the cathode. The anode, usually of carbon or graphite, is disposed centrally in the anode compartment.

In the mercury cathode cell, the cathode is a flowing stream of mercury which flows along a solid metal base connected to the negative pole of a power source. The anode, again of carbon or graphite. is spaced from the mercury cathode and, as electric current flows, the sodium or like alkali metal is evolved and collected in the mercury as an amalgam which is removed from the cell. Outside the cell the mercury amalgam is contacted with water in a "denuder" to remove the sodium as sodium hydroxide solution and the mercury is then recycled.

ln operating each of the above-described cells one is confronted with a common problem; namely, that during the course of the electrolysis, the carbon or graphite electrode erodes and/or decomposes. Consequently, as the electrodes wear away or erode, the spacing between the electrodes increases with resulting increase in voltage between electrodes. This, together with the reactions which cause degradation of the anode, results in a loss of current efficiency for the production of the desired product. The graphite anodes ultimately must be replaced. In all these cells this erosion increases as the anode current density is increased. At the same time, the trend of operation is toward high-current density to increase the amount of product produced per unit cell. Thus it has become necessary to resort to anodes or bipolar electrodes which remain dimensionally stable and do not erode appreciably over long periods of cell operation.

The present invention is directed to the provision of an improved stable electrode and to electrolytic cells, particularly to cells of the type described above which contain such electrodes as the anode or anodic surface thereof.

Electrodes herein contemplated normally should possess a certain degree of rigidity and, in any event, they must have surfaces which exhibit good electrolytic characteristics. These characteristics, particularly in the case of anodes, include low oxygen and chlorine overvoltage, resistance to corrosion and decomposition in the course of use as anodes in the electrolytic cell, and minimum loss of coating during such use. It is well known that certain metals, metallic oxides, and alloys are stable during electrolysis and have other superior properties when used as anodes. Such metals typically include the members of the platinum group; namely, ruthenium, rhodium, palladium, osmium, iridium, and platinum. These metals are not satisfactory for construction of the entire electrode since, for

example, their cost is prohibitive. Therefore, these metals, metallic oxides, and alloys are commonly applied as a thin layer over a strength or support member such as a base member comprising titanium, tantalum, zirconium, niobium, and alloys thereof. These support members have good chemical and electrochemical resistance to the alkali metal chloride electrolyte and the products of electrolysis, e.g., chlorine, hypochlorite and/or chlorate, but may be lacking in good surface electroconductivity because of their tendency to form on their surface an oxide having poor electroconductivity.

DESCRIPTION OF THE INVENTION The present invention provides an electrode having excellent electrolytic characteristics. The electrode has a coating or exposed surface comprising an oxy-c'ompound including a platinum group metal and an alkalineearth metal, particularly calcium or a rare earth metal such as lanthanum. According to one method of producing the anode of the present invention, a thermally decomposable organic mixture containing a thermally decomposable ruthenium organic-compound and a thermally decomposable calcium organic compound is applied to a conductive, chemically resistant base member. The electrode is heated to decompose and/or to volatilize the organic matter and other components, leaving a deposit of an electroconductive oxy-compound of ruthenium and calcium, probably in the form of preferably calcium ruthenium trioxide, hereinafier called calcium ruthenite (CaRuO,), or calcium ruthenium tetra oxide, hereinafter called calcium ruthenate (CaRuO,), or mixtures thereof. The electrode of the present invention has a low chlorine and oxygen overvoltage. Whereas calcium oxide reacts or is rapidly eroded when used as an anode in the contemplated electrolysis, the my compound of ruthenium and calcium herein contemplated is stable over a long period of time when used as an anode with little or no loss of calcium or ruthenium.

The electrode base is preferably of titanium, and one or a plurality of layers of a mixture of certain thermally decomposable metal compounds such as organic; and inorganic salts of both calcium and ruthenium are applied to the base and decomposed by heating the coated base. Especially useful for this purpose are mixtures of ruthenium resinate and calcium resinate, as well as ruthenium chloride (RuCl,) and calcium formate [Ca( l-iCO,),]. The resulting coating following heating comprises an oxy-compound including ruthenium and calcium and is believed to be in the form of calcium ruthenite (CaRuO although some calcium ruthenate (CaRuO may also be present. Resinates of the type used herein are manufactured by the l-lanovia Division of Englehart Industries. The metallic resinates may be mixed with an organic solvent or diluent, such as terpenes and aromatics, typically oil of turpentine, xylene, and toluene, before being applied to the base member for further increasing adhesion.

As a general rule, the coating is applied as a series of thin layers in order to promote maximum adhesion of the coating to the base. The layers, which are an intimate mixture of the calcium and ruthenium salts, are then heated between coating operations to volatilize or drive off the organic matter, solvent, decomposition products, etc., and form the oxy-compound of the metals as a thin film on the base member.

The exact temperature to which the electrode coating should be heated depends upon the time of heating and temperature at which the calcium compounds and ruthenium compounds decompose. It should be high enough to cause formation of the oxy-compound of the alkaline earth metal and ruthenium, such as the alkaline earth metal ruthenate or ruthenite. Care should be taken to select temperatures and duration of heating that will provide the oxy -compound rather than calcium oxide and the elemental platinum group metal. Typically, the temperature may be in the range of 300 C. to 800 C. for between 10 minutes and 2 hours. The heating step is most advantageously conducted in an atmosphere containing elemental oxygen such as air or other oxygen-inert gas mixtures although an atmosphere of pure oxygen can be used. The oxyocompound thus formed is crystalline or amorphous depending upon the temperature of heating; the higher the temperature and the longer the heating, the greater the crystallinity of the product. Both crystalline, particularly if such crystals are very small in size, and noncrystalline coatings have good electroconductivity. However, products of improved adhesion and conductivity are obtained when care is exerted to maintain the coatings in a state where crystallinity is low. As used herein, low crystallinity will mean an X-ray diffraction pattern of less than 700 percent above background when measured on a Philips Difi'ractorneter under the following conditions: The detector is a sealed proportional counter operated at 35 kv., l milliamperes on the X-ray tube and at 1,000 counts per second full scale. Copper radiation is used and the Philips Difi'ractometer is adjusted as follows: 1 divergence slit, 0.006-inch receiving slit, and 1 scatter slit. The detector is rotated at 2 two theta per minute with a time constant of 2 seconds and the specimen is rotated at 1 per minute.

The organic and inorganic compounds may, if desired, be applied by brushing a coating on the titanium base member or, alternatively, by any other method of application such as spraying or dipping. The electrode must then be heated to a temperature sufficient to drive off the organic and inorganic products and to form the oxy-compound described above.

The present invention is principally directed to a coating of an oxycompound of calcium and ruthenium such as calcium ruthanate [Ca(RuO.)] and calcium ruthenite [Ca(RuO The oxy-compound, preferably, has a ratio by weight of l calcium to between x and y of ruthenium, whereby x may be as low as l but rarely less than 0.25 and wherein y may be as high as 4 but rarely higher than 10. Usually, the ratio is about 1 calcium to 2.5 ruthenium. However, it should be recognized that a platinum group metal or the oxide of a platinum group metal, as well as a limited amount of other impurities, could be present in the oxy-compound. For example, some amorphous titanium dioxide could be present. Also, some calcium oxide could be present when the electrode is initially made; however, such calcium oxide would quickly dissolve once the electrode is placed in operation. Some calcium could also be present as other compounds of calcium such as calcium sulfate.

Other electroconductive oxy-compounds of other alkaline earth metals and ruthenium such as strontium ruthenate [Sr(RuO b], strontium ruthenite [Sr(RuO barium ruthenate [Ba(RuO.)], barium ruthenite [Ba(Ru0,)]. and [Ha Sr,, (RuO )]may be applied to the titanium or other conductive base in lieu of the calcium-ruthenium compound and the thus-coated anode used to electrolyze alkali metal chloride solution.

Although it is preferable to form the oxy-compound in situ from organic compounds of the alkaline earth metal and the platinum metal, the prefonned electroconductive oxy-compound may be applied to the base member, for example, by suspending the oxy-compound in a fluid carrier such as titanium resinate, applying the suspension of the oxy-compound of the alkaline earth metal and ruthenium to the base member, and removing the fluid carrier such as by evaporation. Alternatively, the resinates of barium, strontium or magnesium may be applied to the titanium or like base, together with ruthenium resinate, as described in connection with calcium resinate and ruthenium resinate in example I. Moreover, the oxy-compound can be formed in situ from inorganic compounds of the alkaline earth metal or rare earth metal and the platinum group metal as shown in example ill. The inorganic compounds must be decomposable to form the oxy-compound, for example, by heating. The term alkaline earth metals as used herein includes barium, calcium, strontium, and magnesium. The term rare earth metals as used herein includes lanthanum, cerium, praseodymium, neodymium, promethium, Samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium.

In each of the following examples, the titanium base plate was cleaned or etched prior to application of the coating. The etching process comprised submerging the plate for about 30 seconds in a solution containing 1% HF, said solution being concentrated with respect to HCl The plate was then washed in water and submerged in concentrated l-lCl at about 40 to 50 C. for 1% hours.

EXAMPLE I Electrode l-A was prepared by forming in situ an oxy-compound coating including ruthenium and calcium on a 2-inchby-ZiG-inch titanium plate. The oxy-compound was formed from organic compounds of ruthenium and calcium. The titanium base plate had a thickness of one-sixteenth inch and was thoroughly cleaned by etching. The titanium plate was then coated with a mixture comprising 4 grams of a ruthenium resinate solution and 4.53 grams of a calcium resinate solution. The ruthenium resinate solution contained 4 percent by weight ruthenium. The calcium resinate solution contained 1.4 percent by weight calcium. In preparing electrode l-A, sufficient toluene was added to the mixture to make the concentration of calcium and ruthenium 0.05 molar. Ten layers of such mixture were applied to electrode l-A. Following application of each of the layers l-4 and 6-8, the electrode was heated for 10 minutes at 400 C. Following coatings 5 and 9, the electrode was heated at 500 C. for l0 minutes. After applying the tenth layer, the electrode was heated to 550 C. for 10 minutes, The elec trq M- A was analyzed by X-ray diffraction* and no pattern was found. The coating on the titanium was about 6.0 micro inches in thickness as ruthenium. The coating thickness determination was conducted on a Philips X-ray Spectrograph operating substantially as described in Handbook of X-ray by Kaelble, McGraw-Hill (1967). The electrode I-A was paired with a cathode and operated in a laboratory chlorate cell. The cell contained about 1% liters of a brine solution. The brine solution was maintained at a concentration of to grams NaCl per liter, 500 to 600 grams NaClO per liter, and a pH of 6.8. The following tables show the results obtained:

X-ray diffraction analysis was conducted on a Philips Diffractometer operatira as described in X-ray Diffraction Procedure: by Clug and Alexander. The detector was a sealed proportional counter which was operated at 35 kv. l5 milliamperes on the X-ray tube and at 1,000 counts per second full scale. Copper radiation was used and the Philips Dii'fractometer was adjusted u follows: i divergence slit. 0.006-inch receiving slit, and 1 scatter slit. The detector was rotated at 2' two theta per minute with a time constant of 2 seconds. The specimen was rotated at l per minute.

Current Initial Final" Elapsed Cell tem- Current density cell cell hrs. of perature (amperes) (amperes voltage voltage operation (C.) per square (volts) (volts) foot) "The increase in voltage may be due to polarization of the electrodes and a change in the concentration ofthe cell solutions as a result ofelectrolysis.

The electrode surface area of electrode l-B was reduced to l.5 l.5 inches to afford higher current density from the Power S u p ply and the electrolysis was resumed.

The increase in voltage may be due to polarization of the electrodes and a change in the concentrations of the cell solutions as a result of electrolysis.

An electrode l-B was prepared using a titanium plate lXX %Xl/ 16-inch. Ten coats of the ruthenium resinate-calcium resinate mixture were .applied..-together withvheating as described with. respect to electrode l-A. Electrode [-8- was operated in the chlorate cell for 140 hours at a temperature'of 90 C. The overvoltage ranged between 0.05. and :06of a volt during such operation.

EXAMPLE n The electrodes lI-A and vll-B were .prepared'in a-manner similar to electrodes l-A and I-B'except that the coatingwas an oxy-comoound of ruthenium and strontium. The electrodes were prepared from 2 2%-mch etched t tanium platesz-A' mix- .ture was prepared comprising 4 grams ruthenium resinate and 5.87 grams of strontium resinate. The strontium 'resinate solution contained 7.1 percent by weight strontium aiid -the ruthenium resinate solution contained 4 percent by weight ruthenium. The molar ratio was 3 strontium to l ruthenium. In preparing electrodes ll-AandlFB, the mixture was diluted with toluene to provide a strontium"concentrationof OJ molar. Ten layers of k the respective mixtures were applied to the titanium plates. The electrodes ll-A andll-B were heated .to 400C. for 10 minutes followingsthe application-of layers 1 through 4 and'6 through. 8.- The electrode was heated to 500 C. for 10 minutesfollowirig application of-layers 5gand 9 and to 550 C. for 10 minutes following layer 10. The electrode II-A was analyzed by X-ray'diffraction* andnodefinitep'att rn was nd. creit r s ront um qx slssrrythsniumstx Electrode Il-A had an city-compound coatingthi'ckness delq as mFPi i i am L t misiqj q hs ruthenium. The electrode Il -A was operatedas an mam a laboratory chlorate cell substantially as describedwith'respect to electrode I-A in example I for a period of235 hours at a current density of 500 amperes per square foot. The initial cell voltagewas 3.6 volts and thefinal cell-voltage was 3.95 volts. Electrode 11-13 was operated :in anovervoltage cell substantially as described with respect toelectrode I-B in example I at 500 amperes per square foot and at 90 C. for 145 hours. The initial overvoltage-was 0.04 volt and the final overvoltage was about 0.06 volt.

See example} for desc iption fX-ray difl'raction r etliod 7 EXAMPLE lll hours at 500 amperesper square foot of anode surface. The

cell voltage was 3.45 volts. Electrode lll was removed from the cell and heated at 600 C. for l5 minutes. Electrode III was operated in the laboratory chloratecell for 168 hours at 500 amperes per square foot. The initial cell voltage was 3.6 volts and the final cell voltage was 3.8 volts.

EXAMPLE IV Electrodes lV-A and W4! were prepared by forming an oxy-compound coating on etched titanium plates whose dimensions were 2 2%Xl/l 6 inch. The oxy-compounds were formed from inorganic compounds of the platinum group metal and of the alkaline earth metal. The coating on electrode lV-A was formed by applying 6 coats of a mixture comprising an aqueous solution which was 0.2 molar in ruthenium chloride (RuCl,) and 0.2 molar in calcium formate [Ca(H- (10,),1. Electrode lV-A was heated to 400 C. for 10 minutes following coats I through 3, to 450 C. for 10 minutes following coat 4, to 500 C. for 10 minutes after coat 5, and to 550 C. for H) minutes following the final coat. The electrode lV-A had a coating of an oxy-compound of ruthenium and calcium.

The thickness of the coating was 5.7 micron inchesa'sruthenium. The electrode was 'opera'ted'as an anode'in'a chlorate cell "substantially as described in example I. The current density was 500 amperes per square foot. Thecellvoltage was'3.4

. volts when electrolysis was begun and 3.6' voltsafter 77 hours of operation. The coating-on electrode lV-B was formed by "applying 6 coats of a mixture cornprisingan aqueous solution 1 0.2 molar in 'ruthenium chloride; (RuCl,) "-and 0.2 molar in strontium formate [S'r(l-lCO,),].' The electrode was heated 1 following each coat as described withres'pect to'eleetrode lV-A. The electrode lV-B had a coating thicknesslof 6.9

\ micro inches as ruthenium. The electrodelV B was operated .in a chlorate cell for 2 2 hoursat'a curren't'density of 500 amperes per square foot. The "cell'iioltage" increased from 3.6 to

3.8 volts. Other inorganic alkaline earth-metal compounds and rare earth'm'etal compounds from which the oxy compound could -be formed would 'include oxalates,acetatesiand: nitrates.

Other inorganic platinum group metal compounds which may be used includethe oxalates,'- nitrates, acetates, fol'i'h ltei', car- It bonyls, tricarbonyl chlorides, chlorides, nitroso chlorides, and

- nitrohydroxides.

The'broader aspects of the present invention would' further include'the electroconductiv'e' oxy-cornpouiids ofalkaline earth metals and other platinum group 'metals, such fother I platinum group metals including rhodiuml'palladium. osmium, iridium,"and platinum. Thus, the city-compounds would include the rhodonates. palladatel, osminatesfiridinateg and platinates of calcium, strontium, barium,'; and magnesium.

Oxy-compounds of alkaline earth metals and platinum group metals would specifically include such salts as calcium iridate [Ca(lr0,'b8], strontium iridite tsntlrcol. calcium I rhodate [Ca(Rh0 bh], strontium" rhodite [Sr (Rh0.)],"'and strontium platinite [Sr.(PtO,)]. The oxy-compound could inciude mixed -ia'lkaline earth-metalsand a f'platinurn'grofup metal, for example, salts in the form A',B'(OsO5) wherein'A isfe h r strontium rontium. The

mixed platinurn group metals. The oxy-coinpound could inelude an alkaline "earthmetal; a I platinum group metal, and another metal suchas'BaJiJtO ln eensiminmnw. particularly when'platinuin is present-in'the-oxymompound'care must be taken to avoid reducing the platinum group metal to the elemental metal.

The coating is principally comprisedofthe'dry-compound of an alkaline-earth metal and a platinumgroup nietal. However, it may further include some mixed oxides as well as some elemental platinum group metal. I

Electrodes made according to 'the presentinventio'n are highly suitable for use as anodes in cells used for the electrolysis of aqueous alkali metal chloride solutions, typically diaphragm cells, mercury cathode cells, and chlorate cells such as those sown in U.S. Pat. Nos. 3,400,055; 3,337,453; 3,3l2,6l4; 3,287,250; 3,203,882; 3,l 19,664; 3,l l6,228; 2,897,463;'and 2,7l9,l l7. 7 I i In its broader aspects, the present invention would include use of the electrode for electrolysis of a nonaqueous material .such as a fused salt. A typical example of such use would be in the electrolysis of lithium chloride to produce lithium metal and chlorine gas. I

Although the present invention has been described with reference to the specific details of particular embodiments thereof, it is-not intended thereof, to limit the scope of the invention except insofar as the specific details are recited in the appended claims.

lclaim:

1. an electroconductive anode for electrolysis of a salt which comprises an electrocondu'c'tive base member having an anodically resistant surface comprising an electroconductive oxy-compound of including a platinum group metal and a member selected from the group consisting of an alkaline earth metal, and a rare earth metal. I

2. The anode as, described in claim 1 wherein said alkaline metal is a member selected from the group consisting of rhodium, palladium, osmium, iridium, platinum, and ruthenium.

5. The anodes as described in claim 4 wherein said surface is comprised of a member selected from the group consisting of calcium ruthenate, calcium ruthenite, strontium ruthenate, strontium ruthenite.

6. The anode as described in claim 5 wherein said surface is substantially amorphous;

223 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3, 616, 4-46 Dated October 26, 1971 Inventor-(5) Bernard J.

It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 6, line 69, Claim 1, "an" should be --An--.

Column 6, line 72, Claim 1, "including' should be deleted.

Column 8, line 3, Claim 5, "anodes," should be --anode--.

Signed and sealed this 2nd day of May 1972.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents 

2. The anode as described in claim 1 wherein said alkaline earth metal is a member selected from the group consisting of barium, calcium, strontium, and magnesium.
 3. The anode as described in claim 1 wherein said rare earth metal is a member selected from the group consisting of lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium.
 4. The anode as described in claim 2 wherein said platinum metal is a member selected from the group consisting of rhodium, palladium, osmium, iridium, platinum, and ruthenium.
 5. The anodes as described in claim 4 wherein said surface is comprised of a member selected from the group consisting of calcium ruthenate, calcium ruthenite, strontium ruthenate, strontium ruthenite.
 6. The anode as described in claim 5 wherein said surface is substantially amorphous. 